Ultrasonic imaging systems for imaging the inside of a living body by transmitting/receiving pulsed ultrasonic waves to/from the living body are most commonly used for medical diagnosis.
In diagnostic imaging modalities, particularly in the fields of X-rays and MRI, contrast media have long been used for imaging a blood vessel system and the like. More specifically, contrast media are used to obtain contrast-enhanced images of the structure and distribution of the blood vessel system by administering the contrast medium into the bloodstream in order to conduct highly accurate diagnoses on malignant tumors, infarctions, and other diseases reflected in the blood vessel system.
Contrast media have not heretofore been widely used in ultrasonic diagnosis. In the past several years, however, they have come into widespread use in this field as well, thanks to the advent of a contrast medium based on a formulation obtained by stabilizing microbubbles of a size of the order of microns in some way. During operation, the stabilization utilizes the nature that the microbubbles with a diameter of about one micron vibrate with great amplitude in resonance with the ultrasonic wave of several megahertz that is used for ultrasonic diagnosis, efficiently scatter the ultrasonic waves of this frequency range as a result, and yield a contrast enhancement capability.
Microbubble-based ultrasonic contrast media are characterized by their strong nonlinearity. This is because the microbubbles have the nature that an increase in their volume under negative pressure becomes much greater than a decrease in their volume under a positive pressure of the same amplitude. For this reason, an echo signal that has been scattered by the microbubbles contains the large quantity of second-order harmonic components having twice the frequency of the transmitted signal. In 1992, V. L. Newhouse et al. reported the first scheme of obtaining from the above second-order harmonic components a blood flow Doppler signal which enhances relative contrast with respect to a soft tissue (refer to Non-Patent Reference 1, for example).
Also, P. N. Burns et al. have proposed a pulse inversion method for summing up two echo signals obtained by transmitting/receiving a sound pressure pulse twice using the polarity reversed waveform of the transmitted pulse (refer to Patent Reference 1, for example). The fundamental wave components of the echo signals from a soft tissue whose motion can be ignored are canceled since a 180° phase-rotated signal is added during the summation. Second-order harmonic components, however, grow by a factor of two since a 360° phase-shifted signal is added. Although the necessary number of transmitting operations is doubled, since pulse inversion is based on the principles that allow the fundamental wave components from the soft tissue to be removed without a band-pass filter, a second-order harmonic echo signal can be obtained that is excellent in distance resolution. In addition, as with a microbubble contrast medium in the bloodstream, for a scattering object whose changes in state during the two transmitting/receiving operations cannot be ignored, a fundamental wave echo signal from the scattering object is not completely canceled. This rather suits the current purpose of obtaining the echo image emphasizing the presence of the contrast medium with respect to that of the soft tissue.
Additionally, W. Wilkening has proposed a method of transmitting/receiving a sound pressure pulse an N number of times using the transmitted-pulse waveform rotated in steps of 360°/N in phase angle (refer to, for example, Non-Patent Reference 2). For example, if N=3, echo signals obtained from three transmitting/receiving operations at carrier phase angles of 0°, 120°, and 240°, are summed in this method. According to this proposal, using this method allows components up to the (N−1)th-order harmonic component to be removed. It is also possible to sharply distinguish between signals of different spectral characteristics by filtering each signal during the summation, not by conducting simple summation. Sharp distinction between an echo generated by reflection from a vital tissue and a signal from the contrast medium goes through the following process. First, a signal is acquired by conducting an ultrasonic imaging operation for a phantom split into two spatial regions beforehand. One of the spatial regions is where the contrast medium echo signal is dominant, and the other is where the tissue reflection echo signal is dominant. Next, the filtering coefficient to be used during the summation is determined using the least squares method so as to maximize the difference between the tissue reflection echo signal and the contrast medium signal. According to the proposal, applying such filtering to subsequent signals as well from the living body makes it possible to sharply distinguish between the contrast medium components and the vital reflection echo components.
Umemura has reported a method of discriminating between a contrast medium signal and a vital nonlinear signal by summing up echo signals obtained by transmitting/receiving a pulse three times at carrier phase angles of 0°, 120°, and 240° (refer to Non-Patent Reference 3). However, no description is given of whether a filter is used after the receiving of the three pulses described in Non-Patent Reference 2.
It is known that intentionally superimposing a second-order harmonic component on an ultrasonic transmission waveform makes it possible to enhance or suppress the vibration, growth, and collapse of microbubbles in a living body or liquid (refer to Non-Patent Reference 4).
Non-Patent Reference 1: 1992 IEEE Ultrasonics Symposium Proceedings, pp. 1175-1177    Non-Patent Reference 2: 2001 IEEE Ultrasonics Symposium Proceedings, pp. 1733-1737    Non-Patent Reference 3: 2003 IEEE Ultrasonics Symposium Proceedings, pp. 429-432    Non-Patent Reference 4: 1996 IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, vol. 43, no. 6, pp. 1054-1062    Patent Reference 1: U.S. Pat. No. 6,095,980